I did the same thing so I am going to share my experiation with you.

You can change your tourist visa to student visa while you are in US. First go to the university and they will tell you whatever you need to do. For this purpose, you need your academic certificates, TOEFL, IELTS( if possible) otherwise you have to take campus placement test or you can take toefl exam while you are in US, Bank statement to show them that you  are capable of paying tution fee and accomodation fee while you are in US and one explanation letter of your intension why you want to change your tourist visa to student visa. After showing all these proof, the university will issue new SEVIS and I-20. After getting all these documents, you can apply to USCIS or university itself will apply to USCIS on the behalf of yourself if you talk to them.

For all these purpose, you have to pay almost 500 dollar and it takes 3 to 5 months to change your status. One important thing is that please do not apply immediately after coming to US. The suitable time to apply for changing your status is 45 days after arrival to US or 45 days befor your visa expiration date.

Finally, you have to remember that you can stay in the US legally as long as you maintain your status but if you leave US you cannot re-enter the US with old visa.You must take new student visa to re-enter the US.

I hope this will help you.

Best of luck in advance.